Field control in imaging systems

ABSTRACT

An ionographic imaging system in which electrons are emitted from a photoemitter in response to impingement of X-rays, cross a gas-filled gap, are multiplied by secondary ionization, and the resulting charged particles are attracted toward an electrode of polarity opposite to that of the photoemitter and having an insulating surface thereon, wherein a substantially constant electric field is maintained in the gap during operation.

United States Patent 1 J virblis et a1.

1 1 FIELD CONTROL IN IMAGING SYSTEMS [75] Inventors: Alex E. Jvirblis; Walter Roth, both of La Jolla, Calif.

[73] Assignee: Diagnostic Instruments, Inc., San

Diego, Calif.

[22] Filed: Jan. 7, I974 [21] Appl. No.: 431,020

Related U.S. Application Data [63] Continuation of Ser. No. 158,172, June 30, 1971,

[451 July 22,1975

3,603,790 9/1971 Cleare 355/17 3,604,925 9/1971 Snelling 250/326 3,638,016 1/1972 Frank r r 250/326 3,660,656 5/1972 Frank r r r i 317/262 A 3,699,335 10/1972 Giaimo 317/262 A Primary ExaminerEli Lieberman Assistant Examiner-D. C. Nelms Attorney, Agent, or Firm-Koenig, Senniger, Powers and Leavitt 15 7] ABSTRACT An ionographic imaging system in which electrons are emitted from a photoemitter in response to impingement of X'rays, cross a gas-filled gap, are multiplied by secondary ionization, and the resulting charged particles are attracted toward an electrode of polarity opposite to that of the photoemitter and having an insulating surface thereon. wherein a substantially constant electric field is maintained in the gap during op 15 Claims, 7 Drawing Figures [56] References Cited UNITED STATES PATENTS 2,825,814 3/1958 Walkup 250/326 3,057,997 10/1962 Kaprelian 250/315 3,470,417 9/1969 Gibbons 250/326 L11 POWER SUPPLY 27 L27 PROGRAMMER PATENTEDJUL 22 ms SHEET 2 BF 2 TIME (SEQ) TIME (SEQ) FIELD CONTROL IN IMAGING SYSTEMS This is a continuation of application Ser. No. 158,172 filed June 30, l97l, now abandoned.

BACKGROUND OF THE INVENTION The general process known as ionography involves making X-ray images without the utilization of silver halide film. The basic process was disclosed by E. L. Criscuolo, in NAVORD Report 4033 of July 6, 1955, U.S. Pat. No. 2,900,515 ofAug. I8, 1959 to E. L. Criscuolo et al, in an article by R. A. Youshaw and J. A. Holloway in Nondestructive Testing, Vol. 17, September-October, page 297, 1959) and by K. H. Reiss in Z. Angew Physik, Vol. 19, page I (1965). This process includes the utilization of two parallel plate electrodes. A dc. voltage is applied across the gap between the electrodes such that one is a positive electrode or anode and the other is a negative electrode or cathode. When the positive electrode is nearest the X-ray beam, it must not absorb much of the X-ray beam. It has affixed to it an image receiving sheet which may be transparent or opaque but must be an electrical insulator such as a thin sheet of a plastic film or the like. The negative electrode has a thin film or layer of a material which is an efficient absorber of X-rays. In the aforementioned Reiss reference, a heavy metal such as lead, was utilized as an absorber of the X-rays and was, in effect, a photoemitter. The image receiving insulator on the anode and the photoemitter layer on the cathode face each other across the gap between the electrodes with the object being examined disposed on either the outer side of the anode or the cathode, preferably on the outer side of the anode. A quenching gas is flowed or, in some cases, may be stationary in the gap between the electrodes. When an object disposed adjacent the anode is irradiated by X-rays or gamma rays, this electromagnetic radiation is differentially absorbed by the object and passes through the transmissive anode and insulator layer affixed thereto and across the gap to strike the photoemitter where it is strongly absorbed by the photoemitter which, as a consequence, ejects electrons having energies up to many kilo-electron volts. The number of electrons emitted is dependent upon the number of X-rays photons absorbed in that portion, the depth of the absorption, and the photon energy. On leaving the photoemitter surface, the electrons find themselves in the dc. field between the electrodes and travel toward the positive electrode. The quenching gas serves to slow down the electrons so that they will not scatter when reaching the insulator and to increase their number by secondary ionization. Upon arriving at the insulator surface, the electrons, and any negative ions which may have been formed by attachment to components of the quenching gas, are collected in an image configuration forming a latent electrostatic image consisting of negative charges corresponding to elements or portions of the object which are relatively transparent to X-rays, and no charges or fewer charges corresponding to portions or elements of the object which are opaque or relatively opaque to X-rays. This latent image is then made visible by development or by cathode ray tube display techniques.

One of the limitations on the sensitivity of the foregoing ionographic process is that, during an X-ray exposure while operating in an amplification region of a Townsend curve (electron current vs. electric field across the gap), as negative charges are deposited on the insulator they retard the arrival of negative charges subsequently produced during the same exposure. The practical effect of this is to lower or reduce the electric field across the gap so that the operating point on the Townsend curve falls to a point of low amplification. This also leads to degradation of contrast of the image produced. The undesirable efiects of these retarding fields are discussed in our U.S. Pat. No. 3,526,767 which describes imaging techniques and apparatus wherein, as image-forming charges are deposited on the insulator as a result of electrons being produced by the photoemitter, opposite polarity charges are formed adjacent the image-forming charges in an attempt to effect neutralization.

However, prior ionographic imaging apparatus and techniques were not capable of substantially completely compensating for the effect of said retarding fields or maintaining a substantially constant electric field in the gap between the electrodes as the latent image charges were deposited on the insulator.

SUMMARY OF THE INVENTION Thus it is an object of this invention to provide methods and apparatus which substantially completely compensate for the retarding field effect of deposited charges on an insulative surface in an ionographic process.

Another object of this invention is to provide methods and apparatus which prevent degradation of contrast of an image produced through an ionographic process and improve the sensitivity and dynamic range thereof.

The above and other objects of this invention are accomplished by maintaining a substantially constant electric field across a gap between the plate electrodes in an ionographic imaging system. The foregoing may be accomplished, for example, by increasing the potential applied to these electrodes as the charges are deposited on the insulative surfaces. In one embodiment of this invention the voltage or potential between or across the electrodes is increased as a programmed function of time during exposure. This potential may also be increased successively by a series of individual adjustments during exposure or by a series of short exposures, each preceded by an appropriate increase in voltage. Preferably, however, incipient changes in the electric field in the gap are sensed and the potential applied to these electrodes by a power supply is increased as a function of these incipient changes. For example, this may be accomplished by sensing the magnitude of current flowing between the electrodes during exposure by connecting an impedance in a series relation therewith and utilizing the voltage drop across the impedance as a feedback signal to control the output voltage of the power supply, the output voltage varying as an inverse function of the feedback signal. In still another embodiment of this invention a grid is interposed in the space between the electrodes and a potential is applied between the grid and the cathode to maintain substantially constant the field in the gap between the grid and the photoemissive surface of the cathode.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. 1 is a schematic view of an ionographic apparatus;

FIG. 2 is a Townsend curve of discharge current vs. field strength in the gap between the electrodes during operation of the ionographic apparatus of FIG. 1',

FIG. 3 IS a block diagram of a first embodiment of this invention for maintaining a substantially constant electric field between the plates of an ionographic system during operation thereof;

FIG. 4 is a block diagram of a second embodiment of this invention for maintaining a substantially constant electric field between the plates of an ionographic system during operation thereof;

FIG. 5 is a block diagram of still another embodiment of an ionographic system of this invention;

FIG. 6 is a graphical representation of discharge current as a function of time without utilizing the present invention and in accordance with the prior art; and

FIG. 7 is a graphical representation of discharge current as a function of time utilizing the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS:

FIG. 1 schematically depicts apparatus for performing an ionographic process. An object 11 is shown as a step wedge of a homogeneous material which is used to simulate the effect of different thicknesses of a given body upon the produced image. This ionographic apparatus includes a flat positive electrode plate or anode 13 which is formed, for example, of aluminum or beryllium or films of these on nonmetallic substrates. Anode 13 is transmissive to X-rays from an X-ray source 15, or to gamma rays. Thus it is preferred that the metal be in as thin a layer as possible, and may take the form of a thin film coating on a resinous or other suitable flat substrate. In this system, the object 11 is shown disposed adjacent the transmissive electrode or anode l3 and between that electrode and X-ray source 15.

Affixed to the positive electrode 13 is an image receiving sheet 17 which can be transparent or opaque to light but must be an electrical insulator. It, thus, can be paper or thin films or sheets of resinous materials, e.g., polyester film such as that sold under the trade designa tion Mylar" by E. l. duPont de Nemours & Co. (Inc). A negative plate electrode 19 is spaced from positive electrode 13. Cathode'l9 must be extremely flat. To provide this flatness at a low cost, a glass plate can be used having a film coating of conductive metal, such as aluminum or the like. Applied to the negative plate 19 is a thin film 21 of a material which is an efficient absorber of X-rays or gamma radiation. For example, layer or film 21 may be formed of lead or other similar material. The image receiving insulative layer 17 and the X-ray absorbing film 2] face each other across a gap 23. A suitable dc. power source such as represented by a battery 25 is used to apply a dc. potential between the electrodes and provide an electric field in the gap 23.

A gas is maintained flowing through or stationary in gap 23. For example, a insert gas or a halogenated hydrocarbon, such as one sold under one of Freon" trade designation by E. l. duPont de Nemours & Co. (Inc), mixed with a hydrocarbon gas may be used. Alternatively, an inert gas such as argon alone, can be uti lized. When the foregoing apparatus is irradiated by gamma rays or X-rays from source 15, this electromagnetic radiation is first differentially absorbed by the step wedge object 11 and then passes without substantial attenuation through the positive electrode 13, insulative material 17 and the quenching gas in gap 23. The X-rays are then efficiently absorbed by the photoemitter layer 21 which, as a consequence thereof, ejects electrons having many kilo-electron-volts of energy. The number of electrons emitted from any surface area or portion of the photoemitter 21 is dependent upon the number of X-ray photons absorbed in that element, the depth of absorption and the X'ray photon energy. On leaving the photoemitter surface, the electrons find themselves in the dc. field within gap 23 and are attracted to and travel toward the positive electrode 13. One of the main functions of the quenching gas is to slow down the electrons so that they will not scatter when reaching the insulative layer 17. The electrons lose their energy by exciting, dissociating and, in some cases, ionizing or attaching to the quenching gas used. Upon arriving at the insulating layer or surface 17, the electrons or negative ions are deposited and collected in an image configuration forming a latent electrostatic image constituted by areas of negative charges of magnitudes corresponding to the relative X-ray transparency of the respective portions of object 11. That is, under thicker elements or portions of the object which are relatively opaque to X-rays, there is much less charge deposited or collected than under thinner object portions which are less opaque to X-rays.

The iatent electrostatic image thus formed on the insulating layer 17 may be then scanned with an electron beam and, by known video techniques, displayed on a cathode ray tube. Alternatively, the latent image may be made visible and fixed permanently to the insulator by any one of a number of other techniques. One such technique is exposure of the electrostatic imagebearing insulative layer to a cloud of powdered toner particles, charged either positively or negatively, which adhere in image configuration on layer 17 and are then fixed by heating them to their melting point.

In the apparatus and process above described, current flows in the gap 23 between the electrodes only during X-ray exposure and in accordance with the principle or phenomenon known as the Townsend discharge. The electric field in the gap is insufficient to maintain the discharge without the aid of an outside radiation source which must produce electrons in the gap. This Townsend discharge has the characteristic shown by the curve of FIG. 2. At low voltage or field strengths, the current depends upon the mobility of electrons in the gap. At higher voltages or field strengths, all the electrons produced are collected and the current becomes almost independent of the voltage and the curve proceeds through a plateau region. At further increased voltages, or field strengths V/L, the electrons are accelerated by the field and gain enough energy to ionize the gas within the gap and provide a secondary source of electrons. The current then increases exponentially according to the relation:

1. i'-i,,e

In the equation, i is the plateau photocurrent, a which is often referred to as the first Townsend coefficient, is the number of ionizing collisions per centimeter of path in the direction of the field; and L is the gap width in centimeters. Thus, one photoelectrons produces e new electrons in traversing the gap, resulting in an amplification of electrons produced per absorbed X-ray photon. As a result there exists the potential in the aforegoing process for making images with much lower X-ray exposure than is required when operating in the plateau region of the curve.

The dashed portion of the curve shown in FIG. 2 is an extrapolation of the exponential growth of the current as discussed above. However, in practice, as the voltage or field strength V/L increases still further, the curve follows a steeper increase as shown by the solid lines. This has been shown to be the result of the acceleration in the electric field of positive ions formed in the gas and the bombardment of the photoemitter by these ions with the resultant ejection of still more electrons. This provides an additional or another type of exponential amplification of the current. The combined effects of gas ionization and positive ion bombardment result in a current which may be described by the following equation:

where B is called the second Townsend coefficient." It can be seen that when B=0, the equation reduces to a simpler exponential growth described earlier in the equation 1. It is always observed, however, that B greatly exceeds [3. Thus, equation 2 above can be approximate as follows:

When a=l3e the current theoretically becomes infinite. It has been observed experimentally that, at that point, the gap breaks down and a spark occurs. Sparking, however, is detrimental to the photoemitter. Thus, although amplification is to be desired in the foregoing process described above, care must be taken to avoid this sparking.

[n the arrangement in H6. 1 the object and X-ray source are disposed on the same side of the positive electrode and the X-rays are transmitted through the positive electrode 13 and the insulator 17 to the photoemitter layer 21, with secondary emission of electrons occurring and returning through the gap 23 to the surface of the insulative layer 17. This arrangement provides better control on the process, even though the X- rays are initially attenuated to a slight degree in passing through the anode 13 and insulator layer 17 prior to striking the photoemitter 21. Alternatively, the object and Xray source can be disposed adjacent the cathode 19. [n this instance, the X-rays would cause an emission of electrons directly from the photoemitter 21 to the insulative layer 17. However, in that configuration, the photoemitter thickness must be more closely controlled since, if it is too thick, many of the electrons produced will not reach the gap 23; and if it is too thin, insufficient X-ray photons will be absorbed and, consequently, too few electrons will be produced.

One of the limitations on the sensitivity of the abovedescribed process is that during an X-ray exposure, while operating in the amplification region of the Townsend curve of FIG. 2, as electrons are deposited on the insulator 17, they retard the arrival of electrons or other negatively charged particles produced later during the same exposure. This is due to the wellknown phenomenon of repulsion of like charges, and these areas of deposited negative charges establish retarding fields. The effect of these is to lower the effective potential or electric field across the gap 23 so that the opening point on the Townsend curve falls from a point B, (FIG. 2) to a point of lower amplification E During sufficiently long exposures, the discharge current first rises quickly to its initial value and then decays during the exposure as shown in FIG. 6. Additionally, the later arriving electrons are most likely deflected from their normal path in the direction of nonimage areas. This becomes a source of background noise in the image and leads to degradation of contrast.

One preferred embodiment of this invention for overcoming the above-described problems is illustrated in FIG. 3 in which a programmable type of power supply 27 provides an output dc voltage, supplied by leads L1 and L2 to electrodes 13 and 19. The potential thus applied between these electrodes is varied by external means. For that purpose a programmer 29 is interconnected with power supply 27. Programmer 29 may, for example, comprise a ramp function or step function generator or other suitable conventional circuit for generating an empirically predetermined function which increases with time during exposure and which causes the power supply output voltage to be varied accordingly. Power supply 27 may be constituted by a programmable power supply such as is commercially available under the trade designation Model 246" from Keithley Instruments, Inc. lts output voltage may be reshaped by programmer 29 in order to optimize the ionographic image quality. Thus, the dc. voltage applied by power supply 27 to the electrodes 13 and 19 is increased with time so as to maintain the electric field in gap 23 substantially constant at a level as indicated at E, in FIG. 2 and substantially completely compensate for the effect of the retarding fields formed by the deposit of negative charges on insulative surface 17 during exposure.

Another method for overcoming the retarding fields is to make a series of short exposure with readjustment of the potential, or voltage applied to the electrodes, after each exposure to bring the field in the gap back to its initial value E until the total required exposure is made, rather than making one normally long exposure. Such stepwise adjustment, or series of individual adjustments, may, for example, comprise manual adjustment of a variable power supply used in place of fixed power supply 23. However, this method of the invention is time-consuming and is not the most practical method of carrying out the invention. Alternately, manual adjustment of the voltage applied between the electrodes may be made without interrupting the radiation, but for short total exposures it would be somewhat difficult accurately and precisely to increase the applied voltage to maintain the field in the gap at its initial value E Another embodiment of this invention which substantially completely compensate for the efiect of the retarding fields is illustrated in FIG. 4. In this instance an incipient decrease on the electric field in gap 23 is sensed and the dc. voltage or potential applied between electrodes 13 and 19 by high voltage power supply 27 is automatically increased as a function of this incipient field change. This change is sensed in this embodiment by connecting an inpedance, resistor 31, in series between the power supply 27 and the electrodes. As the discharge current flowing in gap 23 during exposure also flows through resistor 31, the magnitude of this discharge current is thus sensed. The voltage drop across this resistor is utilized as a feedback signal applied via a lead 30 to the variable or controllable power supply 27 thereby to control the output voltage thereof and vary it as an inverse function of the feedback signal. Thus as the discharge current during exposure incipiently decreases, the feedback signal voltage developed across retainer 31 incipiently decreases which in turn increases the output voltage of supply 27 to maintain substantially constant the discharge current through the gap and thereby maintain the field in gap 23 substantially constant at a value such as represented in E,, thus substantially completely compensating for the effect of the retarding fields as negative charges are deposited on the receptor or insulator surface of anode 13.

It is to be noted that while the operating point on the Townsend curve of FIG. 2 is maintained substantially constant, preferably at an optimum amplification level, in accordance with the above-described embodiments of this invention, this is based on an average electric field intensity over the entire plate or receptor area since different areas of the plate generally receive different exposures and have different retarding fields. The control of the applied voltage in accordance with this invention produces significant improvements in the sensitivity and the useful dynamic range of known ionographic processes and apparatus which were incapable of maintaining a substantially constant field in the gap and substantially completely compensating for the effect of retarding fields.

The graphical representations of FIGS. 6 and 7 illustrate the operation and advantages of the apparatus and process of FIG. 4 as compared to known ionographic systems in which the electric field in the interelectrode gas is not maintained substantially constant and the field-reducing effects of the retarding fields are not compensated. This was demonstrated by utilizing ionographic apparatus such as schematically shown in FIG. 1, and applying a constant voltage or do. potential to electrodes 13 and 19 in the first instance and in the second instance providing circuitry in accordance with FIG. 4 to apply an automatically controlled variable voltage to these electrodes which increases substantially as an inverse function of the discharge current across the gap. The receptor 17 was a Mylar" polyester film 0.007inch thick and 45inch X6.75inch (200 cm area) with a gap of 0.0 l 4inch. Argon gas was maintained in this gap. The X-ray source used was adjusted to 30 kv peak at 100 ma.

In each instance, a Model 246" power supply, as noted above, was utilized as a power source, but in the first instance the output voltage was initially adjusted to I000 v. which was maintained during exposure of about seconds. In the second instance an electrometer such as that obtained under the trade designation Model 6008' from Keithley Instruments, Inc. was used to develop a feedback voltage for controlling the power supply which was initially adjusted to apply a 900 vdc potential across electrodes 13 and 19 and could rise to a maximum of 1400 vdc in response to the control signal fed back to it by the electrometer functioning as impedance 31. The discharge current across the gap was plotted and recorded in each instance for 5 seconds. FIG. 6 illustrates that, with the voltage applied between the ionographic electrodes held constant at 1000 vdc, this current decayed or fell off quite markedly thereby demonstrating the effect of uncompem sated retarding fields built up on receptor 17. The hatched area under the curve of FIG. 6 is a measure of or representative of the number of charges deposited on insulative surface 17.

FIG. 7 demonstrates the effect of sensing the incipient change of field in the gap and increasing the output voltage of the power supply to maintain this field essentially constant. The voltage automatically was raised to 1400 vdc in continuous response to incipient decrease in the gap current. This voltage reached its upper limit shortly before 2 seconds exposure and was maintained at its 1400 v. limit for 5 seconds. The latter 3-plus second time period (shown by the dashed line) illustrates the nonlinear decay of the field as retarding fields on receptor 17 were permitted to remain uncompensated for this terminal portion of the test during which the voltage applied across the electrodes was not permitted to rise further. Thus during the more significant portion of this FIG. 7 curve (nearly the first L seconds) the discharge current was maintained substantially constant rather than decaying as shown in FIG. 6. The re sult is that it took only a litte over one-third of the time (1.75 seconds) to produce an area under the curve of FIG. 7 equal to the entire area under the curve in FIG. 6. As noted above, these areas are a measure of the charge deposited on the insulating layer. As a result, it can be seen that at the end of about 1.75 seconds, using the process and apparatus of this invention, there has been a sufficient collection of charge to make a proper exposure, equivalent to that which required 5 seconds exposure (FIG. 6) without maintaining the field essentially constant and permitting the retarding fields to remain uncompensated. Thus, within the required 1.75 second period of FIG. 7 the top of the curve is essentially flat, reflecting the achievement of the desired results of this invention.

It should be understood that the current values set forth in FIGS. 6 and 7 are dependent upon the particular apparatus used herein and the particular photoemitter. These current values may change as variables in the process and apparatus change including gap width, photoemitter, area of electrodes, quenching gas and the like.

In accordance with this invention the magnitude of this current flowing between the electrodes may be integrated as a function of time by any conventional means, such as by a capacitor in a RC circuit, and the radiation terminated upon the integrated current reaching a predetermined value, e.g., 1.75 seconds in the example given above.

It is to be understood that the sensing of the field in the gap may be by means other than sensing the change in the current flowing across the gap during exposure and that changes in the field itself or of the actual retarding field build-up may be sensed and the sensed changes automatically utilized as described above in conjunction with FIG. 4 to control the output voltage of the power source.

Another embodiment of this invention is illustrated in FIG. 5 in which a fine metal wire mesh grid 33 is interposed between the electrodes 13 and 19. This grid may be so fine that any image pattern it might form on the receptor surface of anode 13 either is not resolvable by the observer after image development or is tolerable. Altemately, grid 33 may be moved laterally relative to the electrodes during exposure, similar to the Bucky grid, so that there will be no significant developable grid image pattern. In this embodiment the electric field in gap 23A between the grid and the cathode is maintained substantially constant during exposure by applying an electrical potential between grid 33 and the photoemissive surface of cathode 19. The total output voltage of power supply 27 is applied via leads L1 and L2 between electrodes 13 and 19 and across a potentiometer 35. By connecting the movable or wiper contact of the potentiometer to grid 33, any d.c. voltage up to or equalling the output voltage of the power supply may be applied to grid 33. Preferably grid 33 may be made electrically common with anode 13 by moving the potentiometer wiper to its top position, or by simply eliminating the potentiometer and electrically interconnecting both the grid and anode to L1. Any other conventional means may be used to provide this grid potential such as a separate power supply. The effect of the retarding fields built up on the receptor surface of the electrode 13 during exposure is substantially obviated by maintaining constant the voltage between grid 33 and the cathode thereby substantially completely compensating for the effect of the retarding fields and maintaining a substantially constant field in gap 23A.

The above description has been particularly directed to the previously described ionographic process wherein electrons were driven from the negative photoemitter toward an insulative substrate on an anode. It has now been found that the polarity of the process can be reversed such that the photoemitter is at the anode and the insulative substrate is on the cathode. In this case, positively charged ions cross the gap and from a positively charged latent image on the substrate adjacent the cathode. Thus, it should be understood that the polarity of the relative electrodes is of no moment as long as the two electrodes are of opposite polarity.

ln view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above apparatus and methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a method of ionographic imaging including directing X-ray or gamma radiation toward an electrode of a first polarity having a pbotoemissive surface thereby to cause electrons to be emitted therefrom, and applying an electrical potential between said electrode and a second electrode of opposite polarity spaced therefrom, said emitted electrons causing charges to be deposited on an opposed insulative surface of said second electrode, said surfaces being spaced apart to define a gap, said depositing of charges forming retarding fields tending to reduce the electric field in the gap and retard the capture and deposit of additional charges on the insulative surfaces; the improvement which comprises increasing the potential applied between said electrodes as a function of incipient reduction of the average electric field in the gap as the charges are deposited on the insulative surface so as to maintain a substantially constant average electric field in said gap and thereby substantially completely compensate for the effect of the retarding fields.

2. The method of ionographic imaging comprising:

positioning an object adjacent one or a pair of electrodes having respectively associated surfaces spaced from each other to define a gap, one of said surfaces being pbotoemissive, the other of said surfaces being insulative;

applying a potential between said electrodes for causing a Townsend discharge in said gap;

exposing said object to X-ray or gamma radiation thereby to cause rays to impinge upon said photoemissive surface and effect emission of electrons therefrom, said emitted electrons causing charges to be deposited on said insulative layer in a latent image configuration of said object, said deposited charges tending to reduce the electric field in said S p;

sensing an incipient change of the average electric field in said gap; and

increasing the potential applied between said electrodes as a function of said incipient change during said exposure thereby to maintain a substantially constant average electric field in said gap.

3. The method of ionographic imaging comprising:

positioning an object adjacent one of a pair of electrode having respectively associated surfaces spaced from each other to define a gap, one of said surfaces being photoemissive, the other of said surfaces being insulative;

applying a potential between said electrodes for causing a Townsend discharge in said gap;

exposing said object to X-ray or gamma radiation thereby to cause rays to impinge upon said photoemissive surface and effect emission of electrons therefrom, said emitted electrons causing charges to be deposited on said insulative layer in a latent image configuration of said object, said deposited charges tending to reduce the electric field in said gap; and

successively increasing the potential applied between said electrodes by a plurality of individual adjustments during said exposure to maintain a substantiaily constant average electric field in said gap.

4. The method of ionographic imaing comprising:

positioning an object adjacent one of a pair of electrodes having respectively associated surfaces spaced from each other to define a gap, one of said surfaces being pbotoemissive, the other of said surfaces being insulative;

applying a potential between said electrodes for causing a Townsend discharge in said gap;

exposing said object to X-ray or gamma radiation thereby to cause rays to impinge upon said photoemissive surface and effect emission of electrons therefrom, said emitted electrons causing charges to be deposited on said insulative layer in a latent image configuration of said object, said deposited charged tending to reduce the electric field in said gap; and

increasing the potential applied between said electrodes as a programmed function of time during said exposure to maintain a substantially constant average electric field in said gap.

5. The method of ionographic imaging as set forth in claim 2 wherein said sensing an incipient change of said average electric field comprises sensing the magnitude of the current flowing between said electrodes and controlling the potential applied to said electrodes automatically to maintain said current substantially constant.

6. The method of ionographic imaging as set forth in claim wherein said sensing the magnitude of the current flowing between said electrodes is constituted by connecting an impedance in series circuit relationship with said electrodes, measuring the voltage drop across said impedance, and feeding back said voltage as a feedback signal to control the output voltage of a programmable power supply providing said potential between said electrodes, said output voltage varying as an inverse function of said feedback signal.

7. The method of ionographic imaging as set forth in claim 5 further comprising the steps of integrating the magnitude of current flowing between said electrodes as a function of time and terminating the radiation upon the integrated current reaching a predetermined value.

8. ln imaging apparatus including a first electrode having a photoemissive surface, a second electrode of opposite polarity to said first electrode and having an insulative surface, said electrodes being spaced apart with the insulative and photoemissive surfaces opposing each other thereby forming a gap, a power supply for applying an electric potential between said electrodes, and a source of electromagnetic radiation adapted to impinge upon said photoemissive surface and cause electrons to be emitted therefrom and the emitted electrons causing charges to be deposited on said opposed insulative surface to form retarding fields which thereby tend to reduce the electric field in said gap and retard further deposit of charges on said insulative surface; the improvement which comprises means for increasing the potential applied between the electrodes as a function of the incipient reduction in the average electric field in the gap as the charges are depos' ited on the insulative surface thereby to maintain substantially constant the average electric field in the gap and substantially completely to compensate for the effect of these retarding fields.

9. lonographic imaging apparatus comprising:

a first electrode having a photoemissive surface;

a second electrode of opposite polarity to said first electrode and having an insulative surface, said electrodes being spaced apart with the insulative and photoemissive surfaces opening each other thereby forming a gap;

a power supply for applying an electric potential between said electrodes;

a source of X-ray or gamma radiation adapted to im pinge upon said photoemissive surface and cause electrons to be emitted therefrom and the emitted electrons causing charges to be deposited on said opposed insulative surface and thereby tend to reduce the electric field in said gap and retard further deposit of charges on said insulative surface;

means for sensing an incipient change in the magnitude of the average electric field in the gap; and

means for causing an increase of the potential applied between said electrodes as a function of the sensed incipient change so as to maintain substantially constant the average electric field in the gap as the charges are deposited on the insulative surface.

10. lonographic imaging apparatus as set forth in claim 9 wherein said means for sensing an incipient change in the magnitude of the magnitude of the average electric field comprises means for sensing the magnitude of the current flowing between said electrodes.

ll. lonographic imaging apparatus as set forth in claim 10 wherein said means for sensing an incipient change in the magnitude of current comprises an impedance connected in a series circuit with said electrodes, said means for increasing the potential between said electrodes comprising a feedback circuit for controlling the power supply as a function of the voltage drop across said impedance.

l2. lonographic imaging apparatus as set forth in claim 10 which further includes means for integrating the current flowing between the electrodes as a function of time and terminating the radiation upon the integrated current reaching a predetermined value.

13. lonographic imaging apparatus comprising:

a first electrode having a photoemissive surface;

a second electrode of opposite polarity to said first electrode and having an insulative surface, said electrodes being spaced apart with the insulative and photoemissive surfaces opposing each other thereby forming a gap;

a power supply for applying an electric potential between said electrodes;

a source of X-ray or gamma radiation adapted to impinge upon said photoemissive surface and cause electrons to be emitted therefrom and the emitted electrons causing charges to be deposited on said opposed insulative surface and thereby tend to reduce the electric field in said gap and retard further deposit of charges on said insulative surface; and

means for causing an increase of the potential applied between said electrodes to maintain substantially constant the average electric field in the gap as the charges are deposited on the insulative surface, said means for causing increase of the potential applied to said electrodes including programmer means for causing the power supply to deliver an output voltage which is an increasing function of time.

14. lonographic imaging apparatus comprising:

a pair of spaced parallel plate electrodes constituting a cathode and an anode;

a photoemissive surface associated with one of said electrodes;

an insulative surface associated with the other of said electrodes, said surfaces being positioned between said electrodes and facing one another to define a 8 power supply means for applying a potential between said electrodes to effect a Townsend discharge in said gap upon exposure to X-ray or gamma radiation of an object adjacent one of said electrodes, the rays passing through the object and impinging upon said photoemissive layer and resulting in deposition of charges in latent image configuration of the object on said insulative layer, the deposited charges tending to reduce the electric field in said gap and to retard further deposition of charges on said insulative layer;

means for sensing an incipient change in the magnitude of the average electric field in said gap; and

means for causing said power supply means to increase the potential applied between said electrodes as a function of the incipient change so as to maintain a substantially constant average electric field in said gap.

15. lonographic imaging apparatus as set forth in claim 14 wherein the potential of said power supply means is varied as a function of a control voltage, said said power supply means, and a feedback circuit for means for sensing an incipient change in the magnitude supplying said control voltage in response to the voltof the average electric field comprising an impedance age drop across said impedance.

connected in a series circuit with said electrodes and 

1. In a method of ionographic imaging including directing X-ray or gamma radiation toward an electrode of a first polarity having a photoemissive surface thereby to cause electrons to be emitted therefrom, and applying an electrical potential between said electrode and a second electrode of opposite polarity spaced therefrom, said emitted electrons causing charges to be deposited On an opposed insulative surface of said second electrode, said surfaces being spaced apart to define a gap, said depositing of charges forming retarding fields tending to reduce the electric field in the gap and retard the capture and deposit of additional charges on the insulative surfaces; the improvement which comprises increasing the potential applied between said electrodes as a function of incipient reduction of the average electric field in the gap as the charges are deposited on the insulative surface so as to maintain a substantially constant average electric field in said gap and thereby substantially completely compensate for the effect of the retarding fields.
 2. The method of ionographic imaging comprising: positioning an object adjacent one or a pair of electrodes having respectively associated surfaces spaced from each other to define a gap, one of said surfaces being photoemissive, the other of said surfaces being insulative; applying a potential between said electrodes for causing a Townsend discharge in said gap; exposing said object to X-ray or gamma radiation thereby to cause rays to impinge upon said photoemissive surface and effect emission of electrons therefrom, said emitted electrons causing charges to be deposited on said insulative layer in a latent image configuration of said object, said deposited charges tending to reduce the electric field in said gap; sensing an incipient change of the average electric field in said gap; and increasing the potential applied between said electrodes as a function of said incipient change during said exposure thereby to maintain a substantially constant average electric field in said gap.
 3. The method of ionographic imaging comprising: positioning an object adjacent one of a pair of electrode having respectively associated surfaces spaced from each other to define a gap, one of said surfaces being photoemissive, the other of said surfaces being insulative; applying a potential between said electrodes for causing a Townsend discharge in said gap; exposing said object to X-ray or gamma radiation thereby to cause rays to impinge upon said photoemissive surface and effect emission of electrons therefrom, said emitted electrons causing charges to be deposited on said insulative layer in a latent image configuration of said object, said deposited charges tending to reduce the electric field in said gap; and successively increasing the potential applied between said electrodes by a plurality of individual adjustments during said exposure to maintain a substantially constant average electric field in said gap.
 4. The method of ionographic imaing comprising: positioning an object adjacent one of a pair of electrodes having respectively associated surfaces spaced from each other to define a gap, one of said surfaces being photoemissive, the other of said surfaces being insulative; applying a potential between said electrodes for causing a Townsend discharge in said gap; exposing said object to X-ray or gamma radiation thereby to cause rays to impinge upon said photoemissive surface and effect emission of electrons therefrom, said emitted electrons causing charges to be deposited on said insulative layer in a latent image configuration of said object, said deposited charged tending to reduce the electric field in said gap; and increasing the potential applied between said electrodes as a programmed function of time during said exposure to maintain a substantially constant average electric field in said gap.
 5. The method of ionographic imaging as set forth in claim 2 wherein said sensing an incipient change of said average electric field comprises sensing the magnitude of the current flowing between said electrodes and controlling the potential applied to said electrodes automatically to maintain said current substantially constant.
 6. The method of ionographic imaging as set forth in claim 5 wherein said sensing the magnituDe of the current flowing between said electrodes is constituted by connecting an impedance in series circuit relationship with said electrodes, measuring the voltage drop across said impedance, and feeding back said voltage as a feedback signal to control the output voltage of a programmable power supply providing said potential between said electrodes, said output voltage varying as an inverse function of said feedback signal.
 7. The method of ionographic imaging as set forth in claim 5 further comprising the steps of integrating the magnitude of current flowing between said electrodes as a function of time and terminating the radiation upon the integrated current reaching a predetermined value.
 8. In imaging apparatus including a first electrode having a photoemissive surface, a second electrode of opposite polarity to said first electrode and having an insulative surface, said electrodes being spaced apart with the insulative and photoemissive surfaces opposing each other thereby forming a gap, a power supply for applying an electric potential between said electrodes, and a source of electromagnetic radiation adapted to impinge upon said photoemissive surface and cause electrons to be emitted therefrom and the emitted electrons causing charges to be deposited on said opposed insulative surface to form retarding fields which thereby tend to reduce the electric field in said gap and retard further deposit of charges on said insulative surface; the improvement which comprises means for increasing the potential applied between the electrodes as a function of the incipient reduction in the average electric field in the gap as the charges are deposited on the insulative surface thereby to maintain substantially constant the average electric field in the gap and substantially completely to compensate for the effect of these retarding fields.
 9. Ionographic imaging apparatus comprising: a first electrode having a photoemissive surface; a second electrode of opposite polarity to said first electrode and having an insulative surface, said electrodes being spaced apart with the insulative and photoemissive surfaces opening each other thereby forming a gap; a power supply for applying an electric potential between said electrodes; a source of X-ray or gamma radiation adapted to impinge upon said photoemissive surface and cause electrons to be emitted therefrom and the emitted electrons causing charges to be deposited on said opposed insulative surface and thereby tend to reduce the electric field in said gap and retard further deposit of charges on said insulative surface; means for sensing an incipient change in the magnitude of the average electric field in the gap; and means for causing an increase of the potential applied between said electrodes as a function of the sensed incipient change so as to maintain substantially constant the average electric field in the gap as the charges are deposited on the insulative surface.
 10. Ionographic imaging apparatus as set forth in claim 9 wherein said means for sensing an incipient change in the magnitude of the magnitude of the average electric field comprises means for sensing the magnitude of the current flowing between said electrodes.
 11. Ionographic imaging apparatus as set forth in claim 10 wherein said means for sensing an incipient change in the magnitude of current comprises an impedance connected in a series circuit with said electrodes, said means for increasing the potential between said electrodes comprising a feedback circuit for controlling the power supply as a function of the voltage drop across said impedance.
 12. Ionographic imaging apparatus as set forth in claim 10 which further includes means for integrating the current flowing between the electrodes as a function of time and terminating the radiation upon the integrated current reaching a predetermined value.
 13. Ionographic imaging apparatus comprising: a first electrode having a photoemissive surfacE; a second electrode of opposite polarity to said first electrode and having an insulative surface, said electrodes being spaced apart with the insulative and photoemissive surfaces opposing each other thereby forming a gap; a power supply for applying an electric potential between said electrodes; a source of X-ray or gamma radiation adapted to impinge upon said photoemissive surface and cause electrons to be emitted therefrom and the emitted electrons causing charges to be deposited on said opposed insulative surface and thereby tend to reduce the electric field in said gap and retard further deposit of charges on said insulative surface; and means for causing an increase of the potential applied between said electrodes to maintain substantially constant the average electric field in the gap as the charges are deposited on the insulative surface, said means for causing increase of the potential applied to said electrodes including programmer means for causing the power supply to deliver an output voltage which is an increasing function of time.
 14. Ionographic imaging apparatus comprising: a pair of spaced parallel plate electrodes constituting a cathode and an anode; a photoemissive surface associated with one of said electrodes; an insulative surface associated with the other of said electrodes, said surfaces being positioned between said electrodes and facing one another to define a gap; power supply means for applying a potential between said electrodes to effect a Townsend discharge in said gap upon exposure to X-ray or gamma radiation of an object adjacent one of said electrodes, the rays passing through the object and impinging upon said photoemissive layer and resulting in deposition of charges in latent image configuration of the object on said insulative layer, the deposited charges tending to reduce the electric field in said gap and to retard further deposition of charges on said insulative layer; means for sensing an incipient change in the magnitude of the average electric field in said gap; and means for causing said power supply means to increase the potential applied between said electrodes as a function of the incipient change so as to maintain a substantially constant average electric field in said gap.
 15. Ionographic imaging apparatus as set forth in claim 14 wherein the potential of said power supply means is varied as a function of a control voltage, said means for sensing an incipient change in the magnitude of the average electric field comprising an impedance connected in a series circuit with said electrodes and said power supply means, and a feedback circuit for supplying said control voltage in response to the voltage drop across said impedance. 